1. Field of the Invention
The present invention relates to an optical head that is provided to an optical disc device for recording information on an optical disc or reading information from an optical disc, and the optical head being capable of obtaining an appropriate signal by projecting a laser beam to the optical disc.
2. Description of Related Art
Optical discs that can reproduce information when a laser beam is projected are used as recording media for storing information such as pictures and sounds. As such optical discs, a DVD (Digital Versatile Disc) is used, and a BD (Blu-ray Disc) having higher density (more storing capacity) than a DVD becomes available in the market recently. The optical disc device that uses these optical discs as a recording medium reproduces information recorded on the optical disc by rotating the optical disc and by projecting a laser beam to a recording surface of the optical disc.
Usually, a single optical head supports reproduction of information from a DVD and a BD for saving space and cost of the optical disc device. In addition, since laser beams corresponding to a DVD and a BD have different wavelengths, the optical head has laser beam sources that can emit laser beams having corresponding wavelengths. The optical head is provided with an objective lens facing the optical disc, and the laser beam that was emitted from the laser beam sources and passed through an optical system of the optical head is condensed on a recording layer of the optical disc after passing through the objective lens.
When the laser beam passes through the objective lens, aberration is generated in the laser beam. In addition, since the optical disc has a structure including a substrate that is a protection layer disposed on the recording layer, another aberration is generated when the laser beam passes through the substrate. Usually, the objective lens is designed to support one type of optical disc (a BD in many cases), so the aberration generated in the laser beam after passing through the objective lens is canceled with aberration generated by the substrate of the BD. Thus, the laser beam becomes with little wave aberration when it is condensed on the recording layer.
FIG. 6 is a schematic diagram of an optical head. The optical head B shown in FIG. 6 includes a blue color laser beam sources 101 that emits a blue color laser beam, a red color laser beam sources 102 that emits a red color laser beam, a dichroic prism 103 that permits the blue color laser beam to pass through and reflects the red color laser beam, a collimator lens 104 that converts divergent rays into parallel rays, a beam splitter 105 as an optical element that reflects a part of an incident laser beam and permits the rest of the same to pass through, an objective lens 106 that condenses the laser beam at a predetermined position on an optical disc Ds, a photo detector element 107 that detects the laser beam reflected by the optical disc Ds, and a detection lens 108 that condenses the laser beam on the photo detector element 107.
The laser beam emitted from the blue color laser beam sources 101 passes through the dichroic prism 103, enters the collimator lens 104, and is converted into parallel rays when it passes through the collimator lens 104. The laser beam converted into parallel rays enters the beam splitter 105, and a half of the incident light passes through the beam splitter 105 and enters the objective lens 106. The laser beam that enters the objective lens 106 is condensed to be a laser spot that is projected to the optical disc Ds.
In addition, the laser beam reflected by the optical disc Ds goes back to the objective lens 106 and becomes substantially parallel rays, which enters the beam splitter 105. A half of the laser beam that enters the beam splitter 105 is reflected and enters the detection lens 108. Then, it is condensed and enters the photo detector element 107. The laser beam is converted by the photo detector element 107 into an electric signal, and information recorded on the optical disc is detected in accordance with the converted electric signal.
In addition, the optical head B is provided with a diffraction grating 109 that separates the 0 order diffraction light and 1st order diffraction light from the laser beam reflected by the optical disc Ds. The photo detector element 107 has a first photo detecting portion 1071 in the middle and two second photo detecting portion 1072 sandwiching the first photo detecting portion 1071. The 0 order diffraction light of the light diffracted by the diffraction grating 109 is detected by the first photo detecting portion 1071, while the 1st order diffraction light is detected by the second photo detecting portion 1072. The first photo detecting portion 1071 detects an RF signal that is a data signal and a focus error signal. The second photo detecting portion 1072 detects a control signal (e.g., a tracking error signal) for drive control of the optical head B.
However, thickness of a disc substrate, wavelength of a corresponding laser beam, and a numerical aperture NA of the objective lens for a DVD medium are different from those for a BD. Therefore, if an objective lens for a BD is used for condensing the laser beam on the recording layer of a DVD, wave aberration with a large aberration component is generated in the laser beam that is projected to the recording layer of the DVD medium.
If wave aberration with a large aberration component is generated, a condensing point of the laser beam that is condensed on the recording surface of the optical disc (hereinafter referred to as a laser spot) may have a large spot diameter, or a dim circular light (a halo) may be formed around the laser spot. As a result, cross talk or jitter may be increased, and accuracy in reproducing information from the optical disc or recording information on the optical disc may be lowered.
In order to suppress generation of this wave aberration, the following method is adopted in the optical head shown in FIG. 6. A distance between the red color laser beam sources 102 and the collimator lens 104 (including a portion bent by the dichroic prism 103) is adjusted (usually, it is shorter than a distance between the blue color laser beam sources 101 and the collimator lens 104), so that the red color laser beam sources 102 is disposed at a position of a distance that makes the red color laser beam be not parallel rays. A condensing point of the red color laser beam condensed by the objective lens 106 is shifted from a condensing point of the blue color laser beam (the condensing point of the red color laser beam is farther). In this way, since the blue color laser beam sources 101 and the red color laser beam sources 102 are arranged so that their condensing points are shifted from each other, generation of aberration in the laser beam that is condensed on the optical disc Ds is suppressed.
In addition, JP-A-2003-223728 discloses a structure in which a single optical pickup is used for reading optical discs having different substrate thickness, corresponding wavelengths and numerical apertures, and an objective lens having a special shape is provided.
Moreover, JP-A-2001-222838 discloses an invention in which aberration of a laser beam projected to each optical disc is corrected by using a liquid crystal element.
However, since there is a difference between distances of the blue color laser beam sources 101 and the red color laser beam sources 102 from the collimator lens 104 (hereinafter referred to as a light emission point difference), the condensing point where the light beam is condensed by the detection lens 108 is also different between the blue color laser beam and the red color laser beam. More specifically, as shown in FIG. 6, the condensing point of the blue color laser beam is farther than the condensing point of the red color laser beam from the detection lens.
For example, if the first photo detecting portion 1071 of the photo detector element 107 is disposed at the condensing point of the blue color laser beam, the 0 order diffraction light of the blue color laser beam is detected by the first photo detecting portion 1071, and the 1st order diffraction light of the same is detected by the second photo detecting portion 1072. However, the 0 order diffraction light of the red color laser beam becomes dim so that it is not condensed by the first photo detecting portion 1071 and is not detected with sufficient accuracy. In the same manner, the 1st order diffraction light of the red color laser beam is not condensed by the second photo detecting portion 1072 and becomes dim so that it is not detected with sufficient accuracy.
In this way, if the 0 order diffraction light and the 1st order diffraction light projected to the first photo detecting portion 1071 and the second photo detecting portion 1072 are not detected sufficiently, the optical disc Ds cannot be read with sufficient accuracy, and accuracy in drive control of the optical head B is decreased.
In addition, since the invention disclosed in JP-A-2003-223728 uses the objective lens having the special shape, a lot of labor and time are necessary for manufacturing the objective lens having a special shape. As a result, manufacturing cost of the optical pickup (an optical head) increases.
In addition, since the invention disclosed in JP-A-2001-222838 uses the liquid crystal element for correcting the aberration, the liquid crystal element and other components for a drive mechanism that drives the liquid crystal element should be added. Therefore, dimensions and cost of the optical head may be increased.